Commonly used field isolation processes include local oxidation of silicon (LOCOS) and poly-buffered LOCOS (PBL). A thermal oxidation step is performed to form field isolation regions. The thermal oxidation typically allows oxidizing species to diffuse along a pad oxide layer which results in a relatively large encroachment.
Two field isolation processes used to form field isolation with less encroachment include sidewall masked field isolation (SWAMI) and laterally sealed LOCOS. Both processes include forming a recess or trench within the substrate and forming a second nitride layer or nitride spacers. With nitride spacers, neither process completely fills the recess or trench. Therefore, these processes may still cause encroachment to be too large.